Electric vehicles require a battery pack comprising a plurality of individual batteries electrically coupled together to provide the necessary energy/power for the vehicle's drive motor(s). The term "electric vehicle" as used herein is intended to include both "all electric" and so-called "hybrid electric" vehicles. Hybrid electric vehicles differ from "all electric" vehicles in that they also include an internal combustion engine coupled in various ways with the vehicle's electric drive system as is well known to those skilled in the art. Batteries are temperature sensitive in that they can be damaged if too hot and lose performance if too cold. For any particular type of battery there is a desirable temperature window within which that type of battery prefers to operate.
In cold weather it is desirable to heat the battery pack to improve its electrical performance and in warm weather it is desirable to cool the battery pack to protect it. However, thermal management of a battery pack is much more difficult then the thermal management of a single battery. For example, the center of the battery pack can frequently run much hotter then batteries along the periphery of the pack unless precautions are taken to equilibrate the temperature throughout the battery pack. Moreover depending on their state of charge, different batteries in the pack will generate different temperature when the same amount of current passing therethrough. Hence, temperatures will vary at different locations within the pack. Regardless, in cold weather it is desirable to heat the battery pack to improve its electrical performance and in warm weather it is desirable to cool the battery pack to protect it.
Location of a battery pack in an electric vehicle is an important consideration in the design of the vehicle. Because of its typically high mass and the need to have room for occupants within the vehicle, manufacturers have typically located the battery pack as low as possible in the vehicle. Locating a battery pack low in a vehicle can expose the battery pack to road water and the like, especially a battery pack which is housed in an enclosure which is vented through the underside of the vehicle.